1. Field of the Invention
The present invention is directed to a glass fiber forming composition, glass fibers spun therefrom and a substrate for circuit boards including the glass fibers as reinforcing material in a resin layer.
2. Description of the Prior Art
The need for high speed and high frequency information transmission is becoming more and more pronounced with the recent development of sophisticated information systems. In the field of mobile communication by car telephones and personal radios as well as new media of satellite broadcasting and cable television network, there has been an increasing demand of miniaturizing electronic devices and also microwave circuit elements such as dielectric resonators utilized in combination with the electronic devices. The size of the microwave circuit elements is determined in dependance upon the wavelength of all applied electromagnetic wave. It is known that the wavelength .lambda. of the electromagnetic wave propagating through a dielectric body having a dielectric constant of .epsilon..sub..GAMMA. is .lambda.=.lambda..sub.0 /(.epsilon..sub..GAMMA.).sup.0.5 wherein .lambda..sub.0 is propagation wavelength in vacuum. Therefore, the microwave circuit elements can be made more compact when utilizing a circuit board or substrate having a higher dielectric constant. In addition, the use of the circuit board of higher dielectric constant is advantageous in that it acts to concentrate the electromagnetic energy within the board and thereby minimize the leakage of the electromagnetic wave. Ill order to give a high dielectric constant to the circuit board, there have been utilized in the art;
1) to fabricate the circuit board from a resin of high dielectric constant, for example, polyvinylidene fluoride (.epsilon..sub..GAMMA. =13) and cyano resin (.epsilon..sub..GAMMA. =16 to 20); PA1 2) to fabricate the circuit board from a suitable resin and disperse therein inorganic particles of high dielectric constant, for example, TiO.sub.2 and BaTiO.sub.3 particles; and PA1 3) to fabricate the circuit board from a suitable resin and reinforce the same by glass fibers or a glass fiber cloth of high dielectric constant. PA1 1) High dielectric constant [.epsilon..sub..GAMMA. ] of 9 or more at 1 MHz and 25.degree. C.; PA1 2) Low dielectric loss tangent [tan .delta.] of 0.6% or less at 1 MHz and 25.degree. C.; PA1 3) High dielectric constant [.epsilon..sub..GAMMA. ] and low dielectric loss tangent [tan .delta.] can be maintained without causing critical changes in these values even at 100 MHz or more; PA1 4) Superior chemical durabilities such as acid-proof, alkali-proof and water resisting property; and PA1 5) High strain point of about 600.degree. C. PA1 1) acrylic acid such as ester-acrylate, epoxy-acrylate, urethane-acrylate, ether-acrylate, melamine-acrylate, alkyd-acrylate, and silicon-acrylate; PA1 2) multifunctional monomer such as triallyl cyanurate, triallyl isocyanurate, ethylene glycol dimethacrylate, divinyl benzene, and diallyl phthalate; PA1 3) monofunctional monomer such as vinyl toluene, ethylvinyl benzene, styrene, and paramethylstyrene; and PA1 4) multifunctional epoxy.
However, the use of the resin of high dielectric constant poses a problem that it suffers from large dielectric loss tangent (tan .delta.) and unstable dielectric characteristics in high frequency range and is therefore unsatisfactory for use at a high frequency, particularly over 100 MHz.
The circuit board dispersed with the inorganic dielectric particles is likely to have uneven dispersion of the particles leading to correspondingly uneven distribution of dielectric constant on the surface of the board. For this reason, the circuit board of this type is found to be also unsatisfactory. Consequently, the circuit board reinforced by the glass fibers or glass cloth is found desirable. In addition, the glass fiber reinforced circuit board is also found advantageous because of its economy and of easy workability such as cutting and drilling. The conventional glass fiber reinforced circuit board normally utilizes a glass cloth made of E-glass which is composed of SiO.sub.2, Al.sub.2 O.sub.3 and CaO and exhibits less dielectric constant of about 6 to 7. In place of the E-glass, there have been proposed lead glass of rather high dielectric constant. For example, lead glass consisting of 72 wt % of PbO, 26 wt % of SiO.sub.2 and 1.5 wt % of B.sub.2 O.sub.3 and 0.5 wt % of K.sub.2 O shows a dielectric constant of 13.0 sufficient to fabricate the circuit board of desired dielectric characteristics. However, such lead glass composition is found difficult to be spun into fibers of 7 to 9 .mu.m in diameter since PbO will evaporate violently at the time of melting to thereby become less uniform in composition and therefore frequently bring about breakage of thread or fibers in the spinning process. In addition, the lead glass composition is not suitable for forming a glass cloth of fibers for use in the circuit board, since the lead glass composition has inherently low strain point and therefore easily deteriorates in a heating process of removing a primary binder which is essential in forming the glass cloth. Thus, the lead glass composition is permitted to be heat-treated only to a limited extent and is therefore not sufficiently removed of the primary binder, which lowers long-term reliability of the circuit board including the glass cloth formed from the lead glass. Further, due to the toxic nature of the lead, the lead glass composition must be handled carefully and is therefore rather inconvenient and not adequate for fabrication of the glass fiber or glass cloth thereof. Furthermore, due to large dielectric loss tangent (tan .delta.), the lead glass composition is not adequate for the circuit board for high frequency use.
Besides, it is generally required for reinforcing the circuit board to utilize a glass composition which shows an excellent chemical durabilities so as not to be damaged or deteriorated in various chemical processes of forming a circuit pattern on the circuit board.